Luminescent compound and method of making the same



LUMINESCENT COMPOUND AND METHOD OF MAKING THE SAME Filed April 9, 1936 LUMINESCENT COATING -Patented June 25,

UNITED s'r'rs 3 i ml, .1.

The present invention relates to a novel fluorescent material useful in the arts generally and particularly useful as a light source within elec' ti-ic lamps and to the preparation of this mate- 5 r all A' particular object of the invention is to provide a novel fluorescent material for use in such lamps and in the arts generally which will have a high efiiciency and along useful life. Another .10 object oi my invention is to provide an effective and inexpensive method oi producing this material. Still other, objects and advantages of the invention will appear from the following detailed specification.

The invention consists in the new and novel composition of matter and in the novel steps of producing the same, as hereinafter set forth and claimed. 7

In the artificial preparation oi luminescent g. substances it has long been known that the best results were obtained by bringing the main luminescent substance into intimate contact with a small amount of another material which serves as an exciting agent, usually in the form of a a solid solution, and that this result was materially aided through the use of a flux, such as sodium carbonate. While many well known luminescent mixtures have been produced by this method, all prior efi'orts to produce an eil'ective a luminescent material containing cadmium milcate according to this procedure have failed, the

luminescence of the product thus produced being so weak and inefllcient that the product was valueless for any practical purpose. This has u been true of all cases where ordinary so-called chemically pure cadmium oxide, silica, manganese dioxide and any of the usual sodium, barium, strontium or calcium fluxes have been sintered together, in accordance with the common practice, to produce a luminescent compound. As a result no hope has been entertained of being able to use cadmium silicate as a source of luminescent light, in spite of the desirable red color of the light emitted thereby.

I have now discovered that this failure to produce a. luminescent product of practical value has been due to the presence oi minute quantities of nickel, iron, cobalt, copper,- lead or the like in the compounds used, and that these traces m of foreign elements have been responsible for the virtual elimination of the luminescence of the resulting product. Consequently the fundamental step of my novel method is a complete elimination of all of these undesired metals from u the product, either by the initial purification of Application Ap t, lttd, wi

(it. can

on. rates the materials employed, or by removal d w the heating incident to the production of the final product. The purification oi the materials themselves is, oi course, a tedious and expensive proceat. so that the latter method is to be preferred, it

ially since I have now discovered that the presence of potassium in the mixture which is calcined to produce the luminous material results in the complete elimination of these undesired metals; or at least of their undesired effects. at it the same time this potassium serves as a flux which very materially speeds up the desired production of a solid solution 0! the active fluorescent elements, with the result that with this novel method a highly efflcient luminescent product is 15 formed within a few". minutes or even seconds. as contrasted with the hours or days which would otherwise he required. It is thus apparent that the potassium or potassium compounds employed have a dual function not possessed by any of the fluxes heretofore used, and that the potassium thus produces a result which'is unattainable with any of these other fluxes.

I have fluthermore discovered that the ratio of the various elements employed is critical and at that the desired result is not produced unless the potassium is kept within certain narrow limits. Thus I have found that first of all the ratio of the cadmium oxide to the silicic acid which is commonly employed should equal .81-1, sirice there it is a rapid decrease in the luminescent eiiect with variations on either side of this value. While this is substantially the molecular ratio required to produce the cadmium silicate CdSirOs it is not certain that this compound is necesit sarily produced, since it is possible that a solid solution of Cd0'2SiO2 is formed. Secondly, the maximum luminescent effect is produced when the manganese dioxide, silver, or other exciting agent and the potassium used together constitute 49 a critical percentage of the total mixture. For example, where manganese dioxide and potas-= sium chloride are used, the total percentage of these compounds for maximum brightness of the luminescent material should be approximately as 3%, the luminescence obtainable falling on to less than a half when this percentage is decreased to 2% or increased to 4%, and pea sharply at 3.1%. Thirdly, the ratio of the potas-' sium chloride to the manganese dioxide is critiso cal and should be approximatelyzfor best results. These ratios and percentages, all by weight, are for the particular compounds mentioned, it being understood that where other compounds or manganese and potassium are em- 55 ployed these figures are varied to keep the proportions of the metals themselves constant, that is, for best results the potassium should be approximately 2.1 of the cadmium oxide, by weight, while the manganese is approximately 1.25% of the cadmium oxide.

The presence of potassium in the resulting product'has been found to be very beneficial in that it produces a crystalline substance which is easily pulverized to produce any desired crystal size, such as desired for the coating of the inner surface of mercury vapor arc lamps and other similar devices. While this pulverization results in a substantial loss in the luminescent effect, as is well known, I have discovered that by heating the crystals to a temperature just below that at which they will again sinter together their luminescent property is restored to the maximum value. Thus this novel process results in the production of a powder of any desired fineness and having highly emcient luminescent properties. In passing it may be noted that this novel process of restoring the luminescent property to powdered material has been found to be effective with all of these luminescent compounds, regardless of the flux used.

The presence of the potassium in my novel luminescent material has also been found to greatly enhance the stability of the product as compared to that which is attainable in any other way, it having been observed that a coating of this material used within a mercury vapor arc lamp is unafiected either by the discharge or by the mercury vapor and hence depreclates in luminous emciency at an extremely low rate.

For the purpose of illustrating my invention I have shown a novel lamp having my new coating on the inner surface thereof in the accompanying drawing, in which Fig. 1 is an elevational view of a lamp having a preferred form, and a Fig. 2 is a sectional view of the lamp tube showing the coating.

The following preferred procedure is illustrative of my invention. Thus according to this novel process cadmium oxide and silica or silicic acid are first intimately mixed together as in a 'ball mill in the proportions of CdO to silica of .811. A potassium carbonate solution is then added to this mixture and stirred to form a thinpaste, after which manganese chloride is added and stirred, the proportions being such that the potassium is substantially 2.1% of the cadmium oxide and the manganese approximately 1.25% of the cadmium oxide, by weight. At this stage the manganese chloride and potassium carbonate interact to produce manganese carbonate (which loses COz' upon heating) and potassium chloride, this particular method of introducing these compounds having been found to give an exceptionally good distribution of these compounds throughout the mass. The mixture is then dried and stirred to further mix it, after which it is heated in air to a temperature of the order of 1100" C. for approximately five minutes or until the fuming stops. During this heating the potassium chloride unites with any traces of i the oxides of lead, nickel, iron, cobalt, copper or fuming during this heating. At the same time the remaining major proportion of the potassium chloride fuses and serves as a flux which causes ing just below the sintering point of the mass,

and then cooled, the resulting powder being ready for use and having an extremely high luminescent emciency.

In some cases several intermediate heatings in the crucible may be added to the above, each heating being at a slightly lower temperature than the one previous thereto, with pulverization following each heating, slightly improved results being obtained in some cases by these added steps, especially where the initial mixing of the various components of the product has not been entirely thorough.

The resulting powder is then used in any desired manner, as for example, by ailixing it to the inner surface of the arc tube I of thelamp shown in Figs. 1 and 2. As there shown this lamp, which is of the order of 1 inch in diameter and some fifty inches long, has a cathode 2 at one end thereof, and a pair of anodes 3 in separate chambers at the opposite end'thereof in accordance with conventional practice where rectifying operation on alternating current is desired, although it is tobe understood that by using a similar cathode 2 in place of the anodes 3 the tube could also be made to operate with a reversing discharge on alternating current. Likewise the device may be operated on direct current with a single anode. Said cathode is illustrated as of the type disclosed in .U. S. Patent 2,009,211, granted July 23, 1935, to James A. St. Louis, but it is to be understood that any of the well known types of cold cathode, such as now used in signs and the like, or hot cathode such as the Pirani or Hull cathodes, for example, are used in place thereof where desired. The anodes are preferably of graphite, or any other suitable material. This lamp may contain any suitable gaseous atmosphere, but I prefer to use argon and mercury, since a discharge in this particular mixture produces slow electrons and radiations, such as the 2537 A. line, which are extremely effective in exciting the luminescent cadmium silicate and likewise produces visible radiations which are complementary to the red of the cadmium silicate, whereby the resultant light emitted by the lamp closely approximates daylight. The argon is usually introduced at a pressure of the order of 2 or 3 m. m. of mercury, while the mercury is preferably restricted to an amount slightly in excess of that which is vaporized at the normal operating temperature, in order to prevent excess deposits of mercury along the tube in operation. Where a mercury containing appendix is employed, however, more mercury is sometimes employed. The luminescent coating is aiflxed to the inner surface of the tube I substantially all the way from the cathode 2 to the anodes Seither by means of a lower melting point glass, as disclosed by Wilford J. Winninghofl in hispending application, Serial No. 511,409, flied Jan. 26, 1931, or by the use of a boric oxide glaze such as described by Eugene Lemmers et al. in their pending application, Serial No. 741,144, filed August 23, 1934, or in any other suitable manner. I

when such a lamp is operated with a one ampere discharge with inductive ballast in a con- 15 proximately 88 volts and the total input 100 watts. Under these conditions the tube itself has a luminous efficiency of better than 35 lumens per watt, while the over-all emcienoy is more than 30 lumens per watt. With lower currents this efliciency is still i'urther increased. 1 have found that my novel coating, due to its unique character, is unailected by the discharge under these conditions, so that the light output varies but little with age, the device thus having a useiul life of many thousand hours.

While I have described my invention by reference to a process wherein the potassium and manganese are added as separate compounds it is to be understood that in some cases they may be added in a single compound, as by the use of potassium permanganate. It is likewise to be understood that other compounds of potassium and of manganese can be employed, and that various other changes, substitutions and omissions, within the scope of the appended claims may likewise be made by those skilled in the art in the steps of the method or in the product without departing from the spirit of my invention.

I claim as my invention:

1. A luminescent material consisting principally of cadmium oxide and silica in substantially the proportions required to produce CdSia-Os together with a small quantity of a metal which serves as a luminescence exciting agent and a small percentage of a potassium compound.

2. A luminescent material consisting of cadmium oxide and silica in substantially the proportions required to produce CdSlaOs intermixed with a manganese compound and a potassium compound, the sum of the manganese and potassium constituting about 3.35% of the cadmium oxide by weight.

3. A luminescent material consisting of cadmium oxide and silica in substantially the proportions required to produce CdSizOs together with potassium and manganese in the form or chemical compounds thereof, said compounds being present in such an amount that the potassium content thereof is substantially 2.1% and the manganese content substantially 1.25% of the cadmium oxide content, by weight, said material being free of impurities of nickel, lead, copper,

cobalt and iron.

TED E. FOULKE. 

